But official statements from the government have not used the words "military hospital", and some in South Africa are questioning whether Mr Mandela is really inside.

"The inference has been all along that he is at the military hospital," the ABC's Africa correspondent Ginny Stein told AM.

"We have seen the defence minister walk out of the military hospital and address the media and give an update on the former president's health - and at no point did they let on that in fact he probably wasn't there at all."

Stein said she had spoken to president Jacob Zuma's spokesman Mac Maharaj, who "pleaded with me as he said he has done with all other media not to say which hospital Nelson Mandela is actually in for his own sake, for his own health, for his recovery".

"But," she added, "I asked, 'Is he in the military hospital,' and he said, 'Well I'm not going to say. I won't confirm one way or the other but I'm asking you please not to say what hospital he is in'."

Mr Mandela has a long history of lung problems. He was diagnosed with tuberculosis while a prisoner on Robben Island.

He became South Africa's first black president in 1994 after 27 years of incarceration.

He left office more than a decade ago and has since retired to his rural childhood village of Qunu in the south-east of the country, but he retains a prominent place in the national psyche.

He retired from public life in 2004 famously saying, "Don't call me, I'll call you".

The revered statesman has not appeared in public since South Africa hosted the FIFA World Cup final in 2010.